This lesson teaches you to

In the transitions framework, animations create a series of frames that depict a change
between the view hierarchies in the starting and ending scenes. The framework represents
these animations as transition objects, which contain information about an animation. To
run an animation, you provide the transition to use and the ending scene to a transition
manager.

This lesson teaches you run an animation between two scenes using built-in transitions
to move, resize, and fade views. The next lesson shows you how to define custom transitions.

Create a Transition

In the previous lesson, you learned how to create scenes that represent the state of
different view hierarchies. Once you have defined the starting scene and the ending scene you
want to change between, you need to create a Transition object
that defines an animation. The framework enables you to specify a built-in transition in a
resource file and inflate it in your code or to create an instance of a built-in transition
directly in your code.

Create a transition instance from a resource file

This technique enables you to modify your transition definition without having to change
the code of your activity. This technique is also useful to separate complex transition
definitions from your application code, as shown in Specify Multiple
Transitions.

To specify a built-in transition in a resource file, follow these steps:

Add the res/transition/ directory to your project.

Create a new XML resource file inside this directory.

Add an XML node for one of the built-in transitions.

For example, the following resource file specifies the Fade
transition:

res/transition/fade_transition.xml

<fade xmlns:android="http://schemas.android.com/apk/res/android" />

The following code snippet shows how to inflate a Transition
instance inside your activity from a resource file:

Create a transition instance in your code

This technique is useful for creating transition objects dynamically if you modify the user
interface in your code, and to create simple built-in transition instances with few or
no parameters.

To create an instance of a built-in transition, invoke one of the public constructors in
the subclasses of the Transition class. For example, the following
code snippet creates an instance of the Fade transition:

Transition mFadeTransition = new Fade();

Apply a Transition

You typically apply a transition to change between different view hierarchies in response
to an event, such as a user action. For example, consider a search app: when the user enters
a search term and clicks the search button, the app changes to the scene that represents the
results layout while applying a transition that fades out the search button and fades in the
search results.

To make a scene change while applying a transition in response to some event in your
activity, call the TransitionManager.go()
static method with the ending scene and the transition instance to use for the animation,
as shown in the following snippet:

TransitionManager.go(mEndingScene, mFadeTransition);

The framework changes the view hierarchy inside the scene root with the view hierarchy
from the ending scene while running the animation specified by the transition instance. The
starting scene is the ending scene from the last transition. If there was no previous
transition, the starting scene is determined automatically from the current state of the
user interface.

If you do not specify a transition instance, the transition manager can apply an automatic
transition that does something reasonable for most situations. For more information, see the
API reference for the TransitionManager class.

Choose Specific Target Views

The framework applies transitions to all views in the starting and ending scenes by
default. In some cases, you may only want to apply an animation to a subset of views in a
scene. For example, the framework does not support animating changes to
ListView objects, so you should not try to animate them during a
transition. The framework enables you to select specific views you want to animate.

Each view that the transition animates is called a target. You can only
select targets that are part of the view hierarchy associated with a scene.

To remove one or more views from the list of targets, call the removeTarget() method before starting
the transition. To add only the views you specify to the list of targets, call the
addTarget() method. For more
information, see the API reference for the Transition class.

Specify Multiple Transitions

To get the most impact from an animation, you should match it to the type of changes
that occur between the scenes. For example, if you are removing some views and adding others
between scenes, a fade out/fade in animation provides a noticeable indication that some views
are no longer available. If you are moving views to different points on the screen, a better
choice would be to animate the movement so that users notice the new location of the views.

You do not have to choose only one animation, since the transitions framework enables you
to combine animation effects in a transition set that contains a group of individual built-in
or custom transitions.

To define a transition set from a collection of transitions in XML, create a resource file
in the res/transitions/ directory and list the transitions under the
transitionSet element. For example, the following snippet shows how to specify a
transition set that has the same behaviour as the AutoTransition
class:

Apply a Transition Without Scenes

Changing view hierarchies is not the only way to modify your user interface. You can also
make changes by adding, modifying, and removing child views within the current hierarchy. For
example, you can implement a search interaction with just a single layout. Start with the
layout showing a search entry field and a search icon. To change the user interface to show
the results, remove the search button when the user clicks it by calling the ViewGroup.removeView() method, and add the search results by
calling ViewGroup.addView() method.

You may want to use this approach if the alternative is to have two hierarchies that are
nearly identical. Rather than having to create and maintain two separate layout files for a
minor difference in the user interface, you can have one layout file containing a view
hierarchy that you modify in code.

If you make changes within the current view hierarchy in this fashion, you do not need to
create a scene. Instead, you can create and apply a transition between two states of a view
hierarchy using a delayed transition. This feature of the transitions framework
starts with the current view hierarchy state, records changes you make to its views, and applies
a transition that animates the changes when the system redraws the user interface.

To create a delayed transition within a single view hierarchy, follow these steps:

When the event that triggers the transition occurs, call the TransitionManager.beginDelayedTransition() method providing the parent view of all the views
you want to change and the transition to use. The framework stores the current state of the
child views and their property values.

Make changes to the child views as required by your use case. The framework records
the changes you make to the child views and their properties.

When the system redraws the user interface according to your changes, the framework
animates the changes between the original state and the new state.

The following example shows how to animate the addition of a text view to a view hierarchy
using a delayed transition. The first snippet shows the layout definition file:

The next snippet shows the code that animates the addition of the text view:

MainActivity.java

private TextView mLabelText;
private Fade mFade;
private ViewGroup mRootView;
...
// Load the layout
this.setContentView(R.layout.activity_main);
...
// Create a new TextView and set some View properties
mLabelText = new TextView();
mLabelText.setText("Label").setId("1");
// Get the root view and create a transition
mRootView = (ViewGroup) findViewById(R.id.mainLayout);
mFade = new Fade(IN);
// Start recording changes to the view hierarchy
TransitionManager.beginDelayedTransition(mRootView, mFade);
// Add the new TextView to the view hierarchy
mRootView.addView(mLabelText);
// When the system redraws the screen to show this update,
// the framework will animate the addition as a fade in

Define Transition Lifecycle Callbacks

The transition lifecycle is similar to the activity lifecycle. It represents the transition
states that the framework monitors during the time between a call to the TransitionManager.go() method and the completion of
the animation. At important lifecycle states, the framework invokes callbacks defined by
the TransitionListener
interface.

Transition lifecycle callbacks are useful, for example, for copying a view property value
from the starting view hierarchy to the ending view hierarchy during a scene change. You
cannot simply copy the value from its starting view to the view in the ending view hierarchy,
because the ending view hierarchy is not inflated until the transition is completed.
Instead, you need to store the value in a variable and then copy it into the ending view
hierarchy when the framework has finished the transition. To get notified when the transition
is completed, you can implement the TransitionListener.onTransitionEnd() method in your activity.